This invention relates to an improved system for the incineration of solid waste materials, and more particularly to the process and apparatus for reducing toxic heavy metals in the fly ash extracted from a solid waste incineration system.
For a number of years, plants for incinerating municipal solid waste have been in operation. Some of these plants convert the solid waste into an energy source for both heating and cooling buildings. Moreover, recently, pollution control units in the form of electrostatic precipitators or dry gas scrubbers are installed downstream of the incinerator furnace to collect and process the effluent flue gas, reducing the waste disposal by-products emitted into the atmosphere to virtually pollution-free water vapor.
An apparatus for incinerating solid waste materials, and particularly municipal solid waste, reduces the total volume of the refuse to approximately one-tenth of its original volume in the form of ash. Thus, a landfill receiving ash from a mass-burn municipal incineration system will last about 10 times longer than if it were filled with the original unburned solid waste.
Conventionally, the fly ash separated by the pollution control device is combined with the ash or "bottom ash" from the incinerator furnace and then transferred to the landfill
The incineration treatment of municipal solid waste has been improved by the pre-sorting of non-combustible materials from the original solid waste refuse before introducing the remaining combustible materials into the incinerator furnace. By utilizing such pre-sorting methods, the efficiency of the incineration of the remaining combustible materials has been improved. The sorted combustible waste burns at a higher and more consistent temperature; the rate of disposal of the refuse increases; and the toxic materials remaining in both the bottom ash and the flue gases, as well as the fly ash, are reduced. Moreover, where a municipal solid waste incineration system is utilized for conversion of some of the waste material into energy for heating a boiler, the efficiency of the boiler has been improved by the pre-sorting of the solid waste refuse.
Furthermore, it has been found in pre-sorting of solid waste material that the ash remaining from the incinerated combustibles is about half the volume of the ash produced in an incinerator furnace in which none of the refuse was pre-sorted. Moreover, the amount of heavy metals in both the bottom ash and the fly ash is significantly reduced in the total incineration system by pre-sorting the refuse to remove such non-combustible materials as metals, both ferrous and non-ferrous, glass, plastics, dirt and heavy organic material.
Nevertheless, even in a mass incineration system for municipal solid waste material including both an efficient pollution control device and a pre-sorting apparatus, it has been found that the fly ash still retains small, but significant, amounts of heavy metals, and particularly lead, cadmium, and mercury, even though the pre-sorting process removes about 75% of the total mercury and cadmium and about 70% of the total lead in the solid waste refuse.
Differences in removal rates for heavy metals can be due to the existence of heavy metals in chemicals that were not physically or magnetically removed with the pre-sorted metals. For example, certain chemicals, such as paint pigments, plastics, inks, and other chemicals contain lead which would enter the incinerator furnace.
Of course, most lead occurs in lead-acid storage batteries. In the case of pre-sorting municipal refuse, most of the batteries are removed. However, there are some incinerator systems which do not utilize pre-sorting.
Although the electrodes in a lead-acid battery are made of pure lead and lead dioxide, and both of these components are inert at ambient or room temperature, nevertheless, when lead is burned or incinerated, it will be converted into lead monoxide at temperatures above 1,020 deg. F. During incineration, both lead and lead dioxide, insoluble in water, are both converted into lead monoxide at the elevated temperatures above 1,020 deg. F. Lead monoxide and water vapor combine to form lead hydroxide at temperatures below 293 deg. F., and lead hydroxide has a cold-water solubility of 155 ppm (parts per million).
Since municipal waste incinerator furnaces must operate at temperatures above 1,800 deg. F., under EPA (Environmental Protection Agency) standards in order to safely dispose of all toxic organics, chemicals, and plastic materials, any lead contained in refuse within such an incinerator will be substantially converted to lead monoxide and subsequently to lead hydroxide. After the materials are cooled, the toxic soluble lead hydroxide will eventually end up in the bottom ash, the fly ash, or the atmosphere. An efficient pollution control system will transfer the metal oxides from the flue gas to the fly ash.
Since the fly ash is combined with the bottom ash and ultimately is deposited in a landfill, the soluble lead hydroxide will be subjected to leaching into the soil and possibly into the underground water streams over a period of time.
Under EPA standards, the ash that tests above 5 ppm (parts per million) for lead is considered toxic and requires special treatment. Ash and fly ash from some state-of-the-art solid waste incineration systems, even including a pollution control unit and a system of pre-sorting or recycling the refuse before entry into the furnace, still register values greater than 5 ppm on the EPC toxicity test. Thus, even though a highly efficient pollution control unit and/or a highly efficient pre-sorting system is utilized in the mass incineration of municipal solid waste, the toxicity test for lead in ash sometimes reveals acceptable values, but more often is exceeded.
Under the present state-of-the-art, some solutions for removal of heavy metals from the ash from the solid waste incineration systems have been proposed, as illustrated in U.S. Pat. No. 4,737,356 issued to O'Hara et al on Apr. 12, 1988. Such prior art processes include the addition of lime, phosphates and other chemicals to the ash or fly ash.
It is also known that the American Society of Mechanical Engineers is proceeding with a research project to vitrify municipal solid waste incinerator ash residue with the intention of determining whether vitrification eliminates the prospect of leaching from ash. However, Applicant is without knowledge of any other information regarding the status of such research project or any details of such project, and particularly is without any knowledge of any details of the steps in carrying out such procedure or any apparatus to be utilized in the research project.